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The answers would be less of a "beauty contest" and have more usefulness if the answers added Rule 5. Please explain why this game is great in one or more sentences. (replay value, good for brief amusement but never gets old, challenging puzzles)
–
bmike♦Apr 27 '11 at 16:08

Also, it would be nice to add a screenshot from the game itself, not from the intro or trailer.
–
sorinOct 15 '11 at 19:53

Portal™ is a new single player game from Valve. Set in the mysterious Aperture Science Laboratories, Portal has been called one of the most innovative new games on the horizon and will offer gamers hours of unique gameplay.

Civilization IV is a turn-based game in which the player builds an empire from very limited initial resources. All standard full-length games begin in 4000 BC with a settler who builds a single city. From there, the player expands an empire while contending with rival nations, using the geography, developing infrastructure, and encouraging scientific and cultural progress. By default, players can win the game by accomplishing one of five goals: conquering all other civilizations, controlling a supermajority of the world's land and population, being the first to land a sleeper ship in the Alpha Centauri star system, increasing the Culture ratings of three different cities to "legendary" levels, or by being declared "World Leader" by winning a popularity election through the United Nations. If the game's clock runs out (by default in the year 2050 AD) with none of these goals fulfilled by any nation, the nation with the highest score is declared the winner.

It's one of the most thoughtful and addictive game I've ever played. Can't wait for version V.

StarCraft II: Wings of Liberty is a military science fiction real-time strategy video game developed by Blizzard Entertainment as a sequel to the award-winning 1998 video game StarCraft. Released on July 27, 2010, StarCraft II is split into three installments: the base game with the subtitle Wings of Liberty, and two expansion packs, Heart of the Swarm and Legacy of the Void.
Set in the 26th century in a distant part of the Milky Way galaxy, the game is built around three species: the Terrans, human exiles from Earth; the Zerg, a race of insectoid genetic assimilators; and the Protoss, a species with vast psionic power. Wings of Liberty focuses on the Terrans, while the expansions Heart of the Swarm and Legacy of the Void will focus on the Zerg and Protoss, respectively. The game is set four years after the events of StarCraft: Brood War, and follows the exploits of Jim Raynor as he leads an insurgent group against the autocratic Terran Dominion. The game includes both new and returning characters and locations from the original game.

Team Fortress® 2 (TF2) is the sequel
to the game that put class-based,
multiplayer team warfare on the map.
This year's most anticipated online
action game, TF2 delivers new
gametypes, a signature art style
powered by Valve's next generation
animation technology, persistent
player statistics, and more.

Unlike other "class-based" games that
offer a variety of combat classes
only, Team Fortress 2 packs a wild
variety of classes which provide a
broad range of tactical abilities and
personalities, and lend themselves to
a variety of player skills.

Play as the flame-throwing Pyro, the
room clearing Heavy, or the Spy, a
master of disguises. Other classes
include: Scout, Sniper, Medic,
Engineer, Demoman, or Soldier.

TF2 features the most advanced
graphics of any Source-based game
released to date - and the most
exciting class-based action ever
created.

An addictive tower-defense-like game by Popcap, maker of a few great casual games.

The game involves a homeowner using many varieties of plants to repel an army of zombies. The homeowner, along with a neighbor called Crazy Dave, must defend the homeowner's house during day or night, in the front yard and the backyard (including the swimming pool), and even on the roof.

By taking the suspense, challenge and visceral charge of the original, and adding startling new realism and responsiveness, Half-Life® 2 opens the door to a world where the player's presence affects everything around him, from the physical environment to the behaviors, and even emotions, of both friends and enemies.

The player again picks up the crowbar of research scientist Gordon Freeman, who finds himself on an alien-infested Earth being picked to the bone, its resources depleted, its populace dwindling. Freeman is thrust into the unenviable role of rescuing the world from the wrong he unleashed back at Black Mesa. And a lot of people he cares about are counting on him.

Half-Life® 2: Episode One is the first in a series of new adventures created by Valve that extends the Half-Life® 2 single player experience.

Stepping into the hazard suit of Dr. Gordon Freeman, you face the immediate repercussions of your actions in City 17 and the Citadel. Rejoin Alyx Vance and her robot, Dog, to once again aid the human resistance in their desperate battle against the totalitarian alien menace of the Combine.

Episode One exposes Alyx's combat skills and knowledge of City 17 as the player battles side-by-side with her in this 4-6 hour adventure of greater density and detail than non-episodic releases.

Half-Life® 2: Episode Two is the second in a trilogy of new games created by Valve that extends the award-winning and best-selling Half-Life® adventure.

As Dr. Gordon Freeman, you were last seen exiting City 17 with Alyx Vance as the Citadel erupted amidst a storm of unknown proportions. In Episode Two, you must battle and race against Combine forces as you traverse the White Forest to deliver a crucial information packet stolen from the Citadel to an enclave of fellow resistance scientists.

The goal of Machinarium is to solve a series of puzzles and brain teasers. The puzzles are linked together by an overworld consisting of a traditional "point and click" adventure story. The overworld's most radical departure is that only objects within the player character's reach can be clicked on.

World of Warcraft, often referred to as WoW, is a massively multiplayer online role-playing game (MMORPG) by Blizzard Entertainment, a subsidiary of Activision Blizzard. It is the fourth released game set in the fantasy Warcraft universe, which was first introduced by Warcraft: Orcs & Humans in 1994. World of Warcraft takes place within the Warcraft world of Azeroth, approximately four years after the events at the conclusion of Blizzard's previous Warcraft release, Warcraft III: The Frozen Throne. Blizzard Entertainment announced World of Warcraft on September 2, 2001. The game was released on November 23, 2004, on the 10th anniversary of the Warcraft franchise.

Torchlight is an action role-playing-game for the PC made by the creators of Diablo, Fate, and the never-released Mythos among other games. Set in the mining town of Torchlight, players will adventure to uncover the mysteries of the Ember mines. It offers a fully featured character leveling system, randomized dungeons, hordes of monsters, and gobs of great loot.

Counter-Strike: Source (officially abbreviated CS:S) is an FPS video game developed by the Valve Corporation. It is a complete remake of Counter-Strike using the Source game engine. As in the original, Counter-Strike: Source pits a team of counter-terrorists against a team of terrorists in a series of rounds. Each round is won either by completing an objective (such as detonating a bomb or rescuing hostages) or by eliminating all members of the opposing team.

"You are a Grey Warden, one of the last of a legendary order of guardians. With the return of an ancient foe and the kingdom engulfed in civil war, you have been chosen by fate to unite the shattered lands and slay the archdemon once and for all. Start your adventure by picking your class as a Human, Elf, or Dwarf. Then after that, it is all up to you. The choices you make guide the path you take through the Ninth Age: The Dragon Age."

(From Bioware, the creators of Baldur's Gate, Neverwinter Nights, Star Wars: Knights of the Old Republic, and Mass Effect – so you know it's going to be fantastic.)

Get set to play the PopCap game that set the standard in casual gaming! Match sparkling gems three at a time to make them burst in showers of color and points. Match four or more to create flashy power gems that boost your score and your mood with brilliant cascades and combos.

Enigmo and Enigmo 2 are respectively
2D and 3D arcade-style computer games
for PC, Mac OS X and the iPhone OS
developed by Pangea Software. They
both involve moving certain substances
into their proper containers. The
music in both games was recorded by
Michael Beckett.

EVE is a Massively Multiplayer Online
Game (MMORPG) that takes place
approximately 20.000 years after our
times in a galaxy on the far end of
the universe. When you join EVE you
assume the role of a freshly graduated
„Capsuleer“, a relatively small group
of elite spaceship pilots capable of
controlling powerful spaceships on
their own from within their capsules.
Capsuleers are often referred to as
"The immortals" due to the fact that
the highly advanced capsules they are
connected to from the inside, are
capable of instantly dowloading their
consciousness to a clone of themselves
in the case of physical destruction.

Altitude is a multiplayer 2-D aerial combat game. Players control one of five customizable aircraft and battle in teams across 2D landscapes. As a player gains experience in battle they unlock new planes and customizations called perks. The game has been described by the press as a cross between Teeworlds, Team Fortress 2 and an RPG. Altitude is primarily an online game with some offline skirmish options and bot support. There are several modes, and plane customizations. Source: wikipedia

Altitude is also available on Steam. It's great for killing 5 minute break, and also for spending few hours. It's very addictive.

Steam is not a game though. As much as I would love to upvote this purely based on the awesomeness of Steam, I encourage you to submit actual games, not distribution platforms :p
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NippysaurusAug 27 '10 at 10:23

Since this list is going to be dominated by more gamer-oriented games, I'm gonna be the voice of casual gaming. Neverball is a great 3d platform game.

The main gameplay of Neverball is based on moving a ball using gravity by tilting the game world, not directly controlling the ball[2] (it is somewhat similar to the game Labyrinth without the holes in the middle). The mouse (or a similar pointing device), keyboard, or joystick tilt the level to help guide the ball to a goal, via an obstacle course, with a set time limit. Moving platforms and other crazy objects get in the way, making levels progressively harder.

X-Moto is a challenging 2D motocross platform game, where physics plays an all important role in the gameplay. You need to control your bike to its limits, if you want to have a chance to finish the most difficult challenges.

I like playing X-Moto while having a short break. Most of the levels are rather short. It's not a games that takes hours to play - but you could invest some time.

is a free open source fast-paced sidescrolling multiplayer-only computer game. The game features simple cartoon-themed graphics and physics, and relies heavily on classic shooter weaponry and gameplay. The controls are heavily inspired by the First-person shooter genre of computer games.

It's seems childish, but it's actually so fun! I wonder how they got it to be only 12MB and use literally like NO system resources. Unusually fun and very stable/efficient game!
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XAleXOwnZXAug 28 '12 at 9:24

Hordes of Orcs is a “Tower Defense”
game in which you must build walls and
lethal towers to defend your village
from the Orcs emerging from, what the
village elders call, “The Glowing
Portal of Really Bad Things That We
Should Have Bricked-Up a Long Time
Ago.” Sadly, the Portal is un-bricked
and the Orcs are very, very hungry. If
20 Orcs make it to your village,
you’re toast.

The game continues on the story from Descent: FreeSpace, once again thrusting the player into the role of a pilot fighting against the mysterious aliens, the Shivans. While defending the human race and its alien Vasudan allies, the player also gets involved in putting down a rebellion of those elements of Vasudan (Hammer of Light) and Human (Neo Terran Front) forces which don't want to cooperate with one another.

The link leads to the Freespace Source Code Project, since the original game was Windows only. Since the source code was released there have been Mac and Linux versions as well as massive improvements to graphics and textures.

The community is alive and well with new campaigns and even full conversions in development.

Set in a futuristic cartoon-like world where rocketlauncher-wielding pigs and lasergun-carrying cyberpunks roam the streets, Warsow is a completely free fast-paced first-person shooter (FPS) for Windows, Linux and Mac OS X.

Speed and movement, that’s what Warsow is all about. Like a true cyberathlete you jump, dash, dodge and walljump your way through the game. Grab those power-ups before your enemy does, plant the bomb before anyone sees you, and steal the enemy’s flag before anyone knows what’s going on!

Counter-Strike: Global Offensive is an online first-person shooter developed by Valve Corporation and Hidden Path Entertainment, who also maintained Counter-Strike: Source after its release. It is the fourth game in the Counter-Strike franchise, excluding Counter-Strike Neo and Counter-Strike Online. It features classic content, such as old maps, as well as brand new maps, characters and game modes. It supports matchmaking and leaderboards
Cross-platform multiplayer was planned between Microsoft Windows, Mac OS X and PlayStation 3 players, but was ultimately limited to PC and Mac due to the differences in update frequency between the systems
Global Offensive adds new weapons and equipment not seen in previous installments, most notably the Molotov cocktail which temporarily covers a small area in fire, making it impossible to pass through without taking damage